This invention relates to optical devices and, more particularly, to an optical multiplexing device that combines or separates light of different wavelengths.
An optical multiplexing device combines light of different wavelengths into a single light beam, or separates the light of different wavelengths from a single light beam. In one approach, the optical multiplexing device includes a series of thin-film bandpass optical filters that reflect the light beam on a beam path between the thin-film bandpass optical filters. Each thin-film bandpass optical filter passes light of a narrow wavelength range, and reflects light of other wavelengths. The light component (also termed xe2x80x9clight channelxe2x80x9d) having a wavelength in the bandpass range may be added to the light beam or separated from the light beam using the thin-film bandpass optical filter of the corresponding bandpass range. The optical multiplexing device of this type is used in wavelength division multiplexing of light signals in optical communications systems.
In those cases where the optical multiplexing device includes a large number of thin-film bandpass optical filters, it is important that the attenuation of the light beam at each of the thin-film bandpass optical filters be as low as possible. One component of the attenuation is the insertion loss as the light beam enters the thin-film bandpass filter. The greater the angle of incidence (as measured from vertical incidence) of the light beam upon the thin-film bandpass filter, the greater is the insertion loss.
It is therefore desirable that the light beam have a low angle of incidence (i.e., nearly vertical incidence) upon the thin-film bandpass optical filter in order to have a low insertion loss. On the other hand, if the angle of incidence is too low, the thin-film bandpass optical filters and their associated light transceivers become too closely crowded together for mechanical compatibility. Some configurations of the optical multiplexing devices have limited the number of thin-film bandpass optical filters that are assembled together in the one device and have adopted special configurations to allow the components to be crowded together, but the result is that the overall size and weight of the optical multiplexing device structure for a large number of light channels are excessively large.
There is a need for an approach for an optical multiplexing device which achieves a low angle of incidence of the light beam onto the thin-film bandpass optical filters, while at the same time allowing a sufficient lateral physical separation between the optical components for mechanical compatibility. The present invention fulfills this need, and further provides related advantages.
The present invention provides an optical multiplexing device capable of performing multiplexing and/or demultiplexing functions. The optical multiplexing device has a reduced angle of incidence of the optical beam onto the optical reflective filters, as compared with other optical multiplexing devices, while maintaining the lateral physical spacing required for mechanical compatibility of the optical components. The reduced angle of incidence leads to a reduced insertion loss and reduced light beam attenuation. The optical multiplexing device additionally may be provided with a temperature-compensating structure that reduces the effects of changes in temperature upon the optical performance.
In accordance with the invention, a multiplexing device operable over a multiplexing wavelength band of light comprises a first optical transmitting plate having a first-plate outer surface and a first-plate inner surface. The first optical transmitting plate is transparent to light within the multiplexing wavelength band. A first-plate optical reflective filter structure is in facing contact with the first-plate outer surface. The multiplexing device further comprises a second optical transmitting plate having a second-plate outer surface and a second plate inner surface. The second-plate inner surface is in a facing-but-spaced-apart relation to the first-plate inner surface with a central medium such as air or vacuum separating the first optical transmitting plate from the second optical transmitting plate. The second optical transmitting plate is transparent to light within the multiplexing wavelength band. A second-plate optical reflective filter structure is in facing contact with the second-plate outer surface. The first-plate optical reflective filter structure and the second-plate optical reflective filter structure are positioned to define a beam path that is alternatively and sequentially reflected from the first-plate optical reflective filter structure and the second-plate optical reflective filter structure. The first optical transmitting plate and the second optical transmitting plate desirably each have an index of refraction to light within the multiplexing wavelength band that is greater than an index of refraction of the central medium to light within the multiplexing wavelength band.
There is typically at least one light transceiver associated with each of the optical reflective filter structures. The light transceiver is a light source in the case where the multiplexing device is a multiplexer, and a light detector in the case where the multiplexing device is a demultiplexer. Preferably, the first-plate optical reflective filter structure and the second-plate optical reflective filter structure each comprises a thin-film optical reflective filter, such as a low-pass or high-pass edge filter, or a central bandpass filter.
In a practical embodiment of the multiplexing device, there is a spacer affixed to, extending between, and separating the first optical transmitting plate and the second optical transmitting plate. A material of construction of the spacer is selected responsive to a temperature coefficient of change of an index of refraction of the first optical transmitting plate, a temperature coefficient of change of an index of refraction of the second optical transmitting plate, a coefficient of thermal expansion of the first optical transmitting plate, and a coefficient of thermal expansion of the second optical transmitting plate. The spacer may therefore be tailored to maintain the appropriate geometry of the multiplexing device as dimensions change with temperature.
The multiplexing device with spaced-apart optical transmitting plates allows the use of various optional features. A rotationally tunable blocking filter may be positioned in the beam path at a location between the first optical transmitting plate and the second optical transmitting plate. A masking aperture may be disposed on one of the first-plate inner surface and the second-plate inner surface and about the beam path. These optional features improve the optical performance of the multiplexing device, by eliminating light of undesired wavelengths and stray light.
In the present approach, the light path through the central medium may have a relatively large angle with respect to the normal (i.e., perpendicular line) to the optical reflective filter structures. This relatively large angle and the spacing between the two optical transmitting plates allow the optical components to be spaced sufficiently far apart laterally to be mechanically compatible. When the light beam enters the higher-index-of-refraction optical transmitting plates, it is refracted to be closer to the normal direction, and consequently to be incident upon the optical reflective filter structure at a more nearly perpendicular incidence. The result is a lower insertion loss than would be the case for a higher angle of incidence upon the optical reflective filter structure. The reduction of the incident angle upon any coating present on the filter also reduces the coating response rate of change of wavelength transmitted versus angle of incidence, thereby allowing greater filter parameter tolerances.